Cryopanels for cryopumps and cryopumps incorporating them



g- 29, 1967 w. H. HOGAN ETAL' 3,338,063

CRYOPANELS FOR CRYOPUMPS AND CRYOPUMPS INCORPORATING THEM Filed Jan. 17,1966 3 Sheets-Sheet 1 CALCULATED N CRYOPUMPING SPEED PER WATT OFREFRIGERATION AT TEMPERATURES BELOW 20K I I I I I0" I0 Io IO'3 IoSTARTING PRESSURE (TORR) Fig.2

CRYOPUMPING SPEED (LlTER/SEC/WATT) 6 INVENTOR. Wulter H. Hogan RaymondW. Moore,Jr.

L44 att ozn ey Aug. 29, 1967 w. H. HOGAN ETAL 3,338,063

CRYOPANBLS FOR CRYQPUMPS AND CRYOPUMPS INCORPORATING THEM Filed Jan. 17,1966 3 Sheets-Sheet E v 28 Q 27 \l Fig. 4

a (1 Q Q W E I 34 I5 l6 l8 1 l ROUGHING l? PUMP OIL Fug. 3

DIFFUSION 1 PUMP IQ, 'VVN- 7 INVENTOR.

Walter H. Hogan Raymond W. Moore AttoFn ey a Aug. 29, 1967 w, HOGAN ETAL3,338,063

CRYOPANELS FOR CRYOPUMPS AND CRYOPUMPS INCORPORATING THEM Filed Jan. 17,1966 3 Sheets-Sheet 3 RADIATION SHIELD TEMPERATURE so L REGION OF 5 comoucnow so I O F (1:125

40 5 X IO 4 Torr L I v 20 l X IO 4 forr REGION OF CRYOPUMPING O 23 /7!2? 28 CRYOPANEL LOCATION F I 2 lIvvEIv'roR.

Walter H. Hogan Raymond W. Moore,Jr.

ABSTRACT OF THE DISCLOSURE The cryopanels for cryopumps disclosed hereinare .formed of a plurality of separate cryopanels which are connected bytemperature discontinuity means serving as thermal resistances. Thesethermal resistances are so designed as to effect automatic engagement ofsucceeding cryopanels as the pressure within the cryopump decreases.This in turn permits optimum use of the refrigerator means.

cryopumps have within the past few years received considerable attentionsince they offer a number of advantages over other means for attainingultrahigh vacuums, e.g., in the pressure range from about 10- to 10-torr. Basically, cryopumping provides a refrigerated surface on whichgas molecules are condensed or sorbed. Vacuum systems incorporatingcryopumps are normally designed so that the gross pumping, i.e., down to10- or 10- torr is achieved through the use of mechanical vacuum pumps.With the attainment of this pressure range, it is then practical tobegin cryopumping and to supplement the cryopumping with a small ion ordiffusion pump which removes the noncondensables, i.e., neon, hydrogen,and helium.

Refen'geration to the cryopanel is normally supplied by circulating acryogenic fluid (liquid hydrogen, or

"liquid helium) in thermal contact with the cryopanel to provide thenecessary cryopumping surface. In such cases, the amount ofrefrigeration delivered can be fairly well controlled by controlling themass flow of the cryogenic liquid. Such cryopumps are now widely used inlarge space simulation chambers where pumping speeds ranging from100,000 to several million liters per second are required. However,cryopumping has not gained much acceptance for applications Wherepumping speeds in the 100 to 10,000 liter per second range are required.This has been due to a lack of suitable mechanical refrigerators and tothe inconveniences inherent in handling liquid hydrogen or liquidhelium. There is, however, a need for vacuum test chambers requiringthese lower pumping speeds and the cryopanels of this invention make itpractical to construct and operate them using recently developed smallcryogenic refrigerators. Thus, the apparatus described herein isconcerned primarily with cryopumps which can be built into andintegrated with small, continuously-operating, cryogenic refrigerators.Miniature cryogenic refrigerators constructed in accordance with theteachings of US. Patents 2,906,101, 2,966,035, and 3,151,466 have beenfound to be particularly well adapted to be built into small spacesimulation chambers and test chamber apparatus requiring extremely goodvacuum. Although such refrigerators may be built in a wide variety ofsizes to deliver a wide range of refrigeration, they can conveniently besized to deliver in the order of 1 watt of refrigeration below 20 K.This represents the refrigeration load normally associated with acryopump having a pump speed of a few thousand liters per second.However, it is not meant to limit the cryopanel of this invention to anysize range of the cryopump.

United States Patent The refrigeration load placed upon the refrigeratorused to cool a cryopanel must be determined both with respect to thatrequired to effect the pumping to reduce the gas pressure to a desiredlevel and to that required to maintain it over a period of time at thatpressure a a given temperature, it has been usual in such cryopumpsv toemploy a cryopanel, the surface area of which represented a compromisebetween that which was optimum for pumpdown and that which was optimumfor sustained operation. The compromise was between a minimum surfacearea which could-be initially cooled to start cryopumping and a maximumsurface area which was desirable when the pressure of the system haddropped and continuous operation at minimum pressure was desired. Thus,cryopumps constructed as integral parts of a refrigerator havepreviously suffered from this need to compromise and have therefore notbeen as efiicient in capturing and immobilizing gas molecules as mightbe desired.

We have now found that it is possible to provide a novel cryopanel for acryopump, incorporating a refrigerator, which eliminates any necessityfor such a compromise in design. Thus, the cryopump of this invention isso constructed as to be able to make optimum use at all times of therefrigeration furnished, while at the same time attaining satisfactorypumping speeds.

It is therefore a primary object of this invention to provide animproved cryo ump incorporating a cryogenic refrigerator which does notrequire the usual compromise in cryopumping surface area. It is anotherobject of this invention to provide a cryopump of the characterdescribed which exhibits pumping speeds up to about 10,000 liters persecond and a high capture coeflicient over its entire pumping range.Other objects of the invention will in part be obvious and will in partbe apparent hereinafter.

The invention accordingly comprises the features of construction,combinations of elements, and arrangements of parts which will beexemplified in the constructions hereinafter set forth, and the scope ofthe invention will be indicated in the claims.

In the apparatus described herein, the necessity for compromise withregard to the cryopanel surface area is eliminated by matching optimumrefrigerator operation with optimum cryopanel performance. This isaccomplished by using a relatively small cryopanel surface area at thehigh gas pressures and then increasing this surface area as the pressureof the surface decreases. This permits a material reduction inrefrigeration load when the cryopump takes over from the mechanicalpump, the reduction being such that the load is not substantiallygreater than for continuous low-pressure operation. The engagement ofadditional cryopanel condensing surfaces into the system as part of therefrigeration load can not be achieved through externally manipulatedmeans for such means would require mechanical connections between thecryopanel within the cryopump and the ambient atmosphere. Suchmechanical connections in turn would provide heat leak paths which cannot be tolerated in an efiicient cryopump.

To effect the required automatic and internally controlled engagement ofone or more additional increments larger than, the condensation surfaceof the primary cryopanel, and one or more thermal resistances betweenthe cryopanels. The thermal resistances provide a temperaturediscontinuity between the primary and secondary panels such that theprimary panel which is in direct thermal contact with the refrigeratoris always at a lower temperature than the secondary panel or panels. Thetemperature difference which defines the temperature discontinuity is afunction of the gas pressure in the cryopump; and when this pressuredrops to a level at which the cryopump load on the secondary panel isacceptable to the refrigerating means, the secondary panel is engaged,cooled and becomes cryopumping.

Although the cryopanel system of this invention will be described interms of its use with a cryogenic refrigerator, it is to be understoodthat it functions equally well when refrigeration is delivered to theprimary panel by contacting it with a circulating cryogenic fluid.

For a fuller understanding of the nature and objects of the invention,reference should be had to be following detailed description taken inconnection with the accompanying drawings in which FIG. 1 is a schematicplot of cryopump pressure versus cryopumping surface area or pumpingspeed for two difierent levels of refrigeration;

FIG. 2 is a plot illustrating the calculated relationship betweencryopumping speed for nitrogen per watt of refrigeration and thestarting pressure of the cryopump;

FIG. 3 is a cross-section of a cryopump incorporating the cryopanelsystem of this invention;

, FIGS. 4-8 are top plan views of four different cryopanel systemconfigurations using a wire as the thermal resistance connecting thecryopanel sections;

FIG. 9 is a side view, partially in cross-section, illustrating the useof a single panel having the thermal resistance the thermal loadrequirements of the cryopanel. This thermal load originates from threemain sourcesradiation, condensation or absorption, and conduction.Radiation is independent of pressure and is normally minimized by theuse of cooled radiation shielding. The thermal load is thereforeprimarily a function of gas pressure-the higher. the pressure, thehigher the thermal load. This means that for a given refrigerationcapacity of a'cryogenie refrigerator in the cryopump the higherpressures in the system (i.e., the greater number of gas molecules whichmust be cooled and immobilized) require smaller cryopanel areas andlower pumping speeds. As the pressure within the system decreases, thecryopanel surface area can be increased.

The problem confronted in the design of a cryopump, in which a cryogenicrefrigerator is incorporated, can best be described with reference tothe schematic plot presented in FIG. 1. Assume that the cryopanel is indirect thermal contact with the coldest end of a small refrigerator.Once the cryopanel is cooled down to temperature, it will provide agiven surface area with which the gas molecules will come in contactthrough their natural motion within the sytem. It will be appreciatedthat the greater the pressure in the system, the more gas molecules arepresent in a given volume so that the number of gas molecules strikingthe cryopanel surface is directly portional to the pressure. Also, thisnumber is proportional 4- to the cryopanel area surface. It is thereforepossible to consider either cryopumping surface area or pumping speed asone parameter of the system as indicated in the ordinate of FIG. 1.Cryopumping must begin at a fixed pressure within the system which isdetermined by the capabilities of the mechanical roughing pump. Sincepumping capacity is limited by the capacity of the refrigerator, thetorr liters which can be pumped is therefore limited. Assume, forexample, that there is one unit of refrigeration available from therefrigerator to cool the cryopanel surface. Then it will be seen fromthe schematic plot in FIG. 1 that a pumping speed of 10 can be attainedor a cryopumping surface area of 10 can be used if the pressure in thecryopump, e.g., at the time the mechanical pump is shut down, is at thearbitrary value of 1. If the pressure in the cryopump at the timecryopumping begins is an order of magnitude greater, i.e., 10, then withthe fixed refrigeration of one unit the cryopumping speed is only one onthis arbitrary scale or the cryopumping surface area must be small.Thus, the need for compromise in the usual design of the cryopanel isimmediately apparent; for although pumping speeds and cryopumpingsurface areas are advantageously fairly large at the very low pressures,they must be fairly small as cryopumping is begun when the pressure isan order of magnitude greater. This may be stated in another way; thatis, of cryopumping is to begin at 10 on this scale and is to achieve arelatively good cryopumping speed of 10, then ten units of refrigerationmust be furnished--a solution which would require a refrigerator tentimes larger than would be required for normal operation at a pressureof one or less.

The apparatus of this invention, by making it possible automatically toengage additional increments of cryopanel surface area as the systempressure decreases thus makes it possible to attain maximum performancethroughout the entire cryopumping period. Returning to FIG. 1, assumethat cryopumping is begun when the pressure in the system is 10 on thearbitrary scale. For a fixed refrigeration capacity of one unit, it ispossible to cool a cryopanel area of one. When the pressure has droppedto one on this scale, then it is possible with the same refrigerationcapacity to cool a cryopanel area of 10 and raise the pumping speed by acomparable factor. Thus, the operational compromise dictated by a fixedcryopanel surface area need no longer be made.

FIG. 2 is a plot of the calculated nitrogen cryopumping speed attainableper watt of refrigeration capacity at temperatures below 20 K. using aliquid nitrogen-cooled radiation shield. It will be seen that one wattof refrigeration would be sufiicient to maintain a nitrogen pumpingspeed of about 100,000 liters per second up to a pressure of 10- torr.However, mechanical pumps used for initial evacuation of a vacuum systemtypically provide a lower limit of 10% torr for a single-stage pump, to10' torr for a two-stage pump in good condition. This would limit thesize of cryopump to take over from the mechanical pump to 200 liters persecond at 10- torr to 15,000 liters per second at 10* torr. Since adirect relationship exists between cryopumping speed and cryopanelsurface area, the desirability of having a small surface area at thehigher pressures becomes apparent. The apparatus of this inventionsupplies the means for increasing the cryopanel surface area withdecreasing system pressure. Thus, when cryopumping is first begun arelatively small cryopanel surface area is engaged and is cooled by therefrigerator. As the gas pressure within the system decreases throughthe immobilization of gas molecules on the cryopanel surface, additionalcryopanel surface area is engaged, thus providing optimum conditionsthroughout the cryopumping cycle.

Before describing the actual operation of the cryopanel system of thisinvention it will be helpful to describe a typical cryopump and toexamine various modifications and embodiments of the cryopanel system.

FIG. 3 illustrates in cross-section what might be com sidered to be atypical cryopump incorporating the novel cryopanel of this invention.The cryopump consists of an upper working or test section joined to alower pumping section 11 through a suitable joining member 12. Withinupper section 10 is a working volume 13 available for experimentalpurposes. Within lower section 11 is a volume 14 containing thecryopumping mechanism. Inasmuch as the evacuation process begins withmechanical pumping, there is supplied a conduit 15 controlled by a valve16 which leads to a mechanical roughing pump (not shown). There is alsoprovided a branch conduit 17 controlled by valve 18 which leads to anoil diffusion or ion pump (not shown). In keeping with normalcryopumping practice, the roughing pump is used to lower the pressurewithin the apparatus to about 10- torr; while the oil diffusion pump isused to remove the residual noncondensables, i.e., neon, hydrogen, andhelium.

The cryopumping mechanism in the arrangement shown in FIG. 3 derives itsrefrigeration from a refrigerator 20 which is shown to be integrallyincorporated into the cryopump. The coldest end 21 of the refrigeratorprovides the refrigeration directly to the cryopanel generally indicatedby the numeral 25. The cryopanel comprises a primary panel 26 which isin direct thermal contact with the cold end 21 of the refrigerator and asecondary panel 27 having an area equal to or greater than the primarypanel and joined to it through a thermal resistance 28 which gives riseto a temperature discontinuity between the panels. As will be seen inthe following detailed description, the primary and secondary panels maytake various forms and the thermal resistance may also take a variety offorms.

In order to reduce the heat leak from the ambient atmosphere to thecryopanel, there is provided around the cold end of the refrigerator andthe cryopanel suitable radiation shielding means which in FIG. 3 takesthe form of a combination of a cylindrical radiation shield 30 andchevron shields 31 positioned above the cryopanel. In order to make theradiation shield more effective, it is cooled with liquid nitrogen whichis circulated through cooling coils 32 which are in turn in directthermal contact with the wall of the radiation shield 30 and with thechevrons 31. Liquid nitrogen, introduced into the cryopump through asuitable inlet conduit 33, circulates first through coils 32 associatedwith the chevrons and then with the cooling coil wound around thecylindrical radiation shield 30. The nitrogen is withdrawn from thesystem through conduit 34.

FIGS. 4-11 illustrate a number of forms which the cryopanel system ofthis invention may take. In FIG. 4 it is seen that the primary panel 26is in the form of a thin circular plate having a single condensingsurface 27 surrounding the circular plate. The thermal resistancejoining these two panels comprises four wires 28. It is preferable,although not necessary, that the wires 28 are formed from a metal whichexhibits an increasing thermal conductivity with decreasing temperature.As an example of such metals we may cite silver which is 99.999% pure,high-purity copper, coalesced copper, and single crystal aluminum.Extremely pure silver, has, for example, a thermal conductivity of 9watts/cm. K at 40 K. and reaches a maximum of about 180 watts/cm. K. atabout 6 K. High-purity copper has a thermal conductivity of about 20watts/cm. K. at 40 K. and reaches a maximum thermal conductivity ofabout 140 watts/cm. K. at about 15 K. (See for example FIG. 10.7 inCryogenic Engineering, by R. B. Scott, P. Van Nostrand Company, Inc.,Princeton, 1959.)

The direct thermal contact between the cold end 21 of the refrigeratorand the small primary cryopanel 26 elfects the necessary cooling of thecondensing surface 22. The temperature of surface 22 is soon low enoughto capture and immobilize a portion of the gas molecules striking it.However, because the secondary panel 27 is connected to the primarypanel 26 only through one or more wires which are thermal resistances,the heat transferred to the secondary panel is transferred to the panelthrough a relatively large temperature difierence. This temperaturedifference maintains the secondary panel at a temperature too high tocondense gas molecules. The heat transferred to the panels is only thatdue to radiation and gas conduction, which may be a small fraction ofthe possible gas condensation load if the secondary and primary panelswere at the same temperature. This condition permits the optimumutilization of the refrigeration delivered and pumping to continue. Asthe pressure decreases within the system, the heat conduction to thesecondary panel is continued and finally a point is reached where thesecondary panel becomes cold enough to capture and immobilize gas, i.e.,it becomes a cryopumping surface 23 (see FIG. 3).

FIG. 12, which is a schematic, somewhat stylized diagram represents atypical situation, using a cryogenic refrigerator such as shown in US.Patent 3,151,466 and helium as the refrigerant. It will be seen that, ata gas pressure of 5 10- torr, the gas conduction heat transfer to thesecondary panel from the radiation shield is enough to maintain thatpanel at a temperature much higher than that of the primary panel,because of the thermal resistance joining the two panels. Under theseconditions, a major heat load is applied to the primary panel as itcaptures and immobilizes gas molecules received from a gas at a pressureof 5 10- torr; a minor heat load by gas conduction from the radiationshield is received by the secondary panel and conducted through athermal resistance to the primary panel. At a gas pressure of l l0 torrthe heat loads on both the primary and secondary panels are reduced andconsequently the temperature difference between them is reduced. In theexample shown, even at a gas pressure of 5Xl0 torr, a sharp temperaturediscontinuity between the primary and secondary panels exists tomaintain the secondary panel at a temperature above which the major loadof cryopumping can be effected. At a pressure of 1 l0 torr, or lower,the heat load to both panels is such that both can be cryopumping. Therewill, however, always be a small temperature discontinuity because ofthe manner in which the panels are thermally connected.

If, in the embodiment shown in FIGS. 4-8, the thermal resistance is awire formed of a metal which displays an increasing thermal conductivitywith decreasing temperature the engagement of the secondary panel ismore rapid at the time such engagement is desired.

The modification shown in FIG. 5 illustrates the use of two secondarypanels, the outer one designates as 27 which is connected to the innersecondary panel by a thermal resistance 28'. This cryopanel systemfunctions in the same manner described for the system of FIG. 4. Thethermal resistances 28 and 28', which are connected in series, may be ofequal or different thermal resistance values.

FIG. 6 shows a construction'in which the circular configuration isreplaced by a polygonal configuration which, of course, may be in theform of a square, rectangle, or other shape.

In FIG. 7 the larger secondary panel 37 is placed at one side of thesmaller primary panel 36 and joined by a wire 38 which may be coiled orformed into a serrated configuration to increase its length withoutmaterially increasing the distance between the primary and secondarypanels.

In FIG. 8, two secondary panels 37 and 39 are connected in parallel tothe primary panel 36 through thermal resistances 38 and 38'. Theresistances are of difierent magnitudes to exhibit different values ofresistance. Thus, these two resistances efiect different cut-in levels.The resistances may also, of course, be formed of two differentmaterials to achieve this result.

FIG. 9 is a cross-sectional detail of another form of the cryopanelsystem of this invention. In this structure the cryopanel 40 is formedof a single piece of material,

e.g., copper, and the thermal resistance is created by milling a deepgroove 44 in the panel 'to'join the primary panel section 41 to thesecondary panel section 42 by a narrow section 43. It will beappreciated that the thermal resistance of the narrow section 43 isconsiderably greater than that exhibited by the total thickness of thecryopanel 40.

FIGS. 10 and 11 illustrate mechanical ways in which thermal contact maybe affected between the primary and secondary panel. In these cases, theatmosphere, which is actually gas at very low pressure, separates thesetwo I panels and forms the thermal resistance. By making actual thermalcontact through the mechanical actuation process, the thermal resistanceof the atmosphere is broken but a temperature discontinuity willcontinue to exist due to the fact that the thermal contact can not beabsolutely perfect.

In FIG. 10 the primary panel 45 is cooled directly by the cold end 21 ofthe refrigerator 20. A bimetallic strip 47, which may be narrow comparedto the lateral dimensions of either of the panels, is permanentlyaffixed to the primary panel 45. The bimetallic strip is formed of twojoined metals 48 and 49. These two metals have different thermalproperties at the temperatures involved. Thus, for example, the metalstrip shown at 49 is one which may have a greater coeflicient of thermalexpansion than metal 48. As bimetallic strip 47 is cooled by virtue ofthe cooling of the primary panel 45 from the center out to itsside, themetal 4 will contract and bend the bimetallic strip downward forcing itinto physical contact with the surface of the secondary panel 46.

In the modification shown in FIG. 11, the primary panel 50 is supportedon a fluid-actuated expansible member 51, e.g., a bellows, which defineswithin it a fluid volume 52. Within the fluid volume 52 is a fluid suchas helium or hydrogen. As the fluid within volume 52 is cooled bycontact with the cold end 21 of the refrigerator, it will, of course,contract in volume and the expansible member will tend to movedownwardly in the direction of the arrows. Contacting members 53attached to the bottom section of the primary panel 50 will then makecontact with the surface of the secondary panel 55 thus providingthermal contact between the two panels. In order to impart a positivedownward motion to the central. primary panel 50, there is provided aspring 56 which is grounded to a suitable ground 57.

Thus, in each of these embodiments and modifications, as the pressure inthe system decreases a point is reached where an additional increment ofcryopanel surface area is engaged. Since the time of such engagementcoincides with that time when the heat load on the panel through gasconduction is decreased, it is possible to furnish the refrigerationnecessary to bring the additional increment of cryopanel surface areadown to cryopumping temperature without increasing the overall load onthe cryogenic refrigerator. This, in turn, means that a fixed amount ofrefrigeration may be used without having to employ a fixed andcompromising cryopanel surface area. The cryopanel of this inventiontherefore makes it possible to provide a cryopump integrated with acryogenic refrigerator. Such a cryopump is particularly well adapted forrelatively small vacuum test chambers and for pumping speeds up to10,000 liters per second.

It will thus be seen that the objects set forth above, among those madeapparent from the preceding description, are efficiently attained, and,since certain changes may be made in the above construction withoutdeparting from the scope of the invention, it is intended that allmatter contained in the above description or shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limitingsense.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindescribed, and all statements of the scope of the invention, which, as amatter of language, might be said to fall therebetween.

We claim: 1. A cryopanel array suitable for incorporation into acryopump, comprising in combination (a) a primary cryopanel in thermalcontact with refrigerating means; (b) at least one secondary cryopanelhaving a condensation surface area at least as great as that of saidprimary cryopanel, and separated from said primary cryopanel by (c)temperature discontinuity means responsive to a decrease in pressurewithin said cyropump whereby said temperature discontinuity meansbecomes sufiiciently small in effect when said pressure has dropped to apredetermined level to permit said secondary cryopanel to attaincryopumping temperature. 2. A cryopanel array in accordance with claim 1wherein said temperature discontinuity means comprises metal wire inthermal contact with said cryopanels.

3. A cryopanel array in accordance with claim 2 wherein said metal ischaracterized by being one which exhibits an increasing thermalconductivity with decreasing temperature.

4. A cryopanel array in accordance with claim 1 having a plurality ofsecondary cryopanels connected in series.

5. A cryopanel array in accordance with claim 1 having a plurality ofsecondary cryopanels connected in parallel with said primary cryopanel.I

6. A cryopanel array in accordance, with claim 5 where'- in saidtemperature discontinuity means separating said secondary cryopanelsfrom said primary cryopanel exhibit different thermal resistance valuesthereby to effect different engagement levels for said secondarycryopanels.

7. A cryopanel array in accordance with claim 1 wherein said temperaturediscontinuity means comprises a thin metal section between said firstand second cryopanels.

8. A cryopanel array in accordance with claim 1 wherein said temperaturediscontinuity means is a small physical gap between said primary andsecondary cryopanels, said physical gap being closable through metallicheat conductive means thereby to effect thermal contact between saidcryopanels to permit said secondary cryopanel to attain said cryopumpingtemperature. a

9. A cryopanel array in accordance with claim 8 wherein said metallicheat conductive means in a bimetallic strip, the metals of which exhibitdiflerent coefiicients of thermal expansion.

10. A cryopanel array in accordance with claim 8 wherein said metallicheat conductive means is fluidactuatable.

11. A cryopump incorporating therein a cryopanel array, comprising incombination (1) housing means defining a working section and a pumpingsection;

(2) cryopumping means located within said pumping section and comprising(a) refrigerating means;

(b) a primary cryopanel in thermal contact with refrigerating means;

(c) at least one secondary cryopanel having a condensation surface areaat least as great as that of said primary cryopanel, and separated fromsaid primary cryopanel by (d) temperature discontinuity means responsiveto a decrease in pressure within said cyropump whereby said temperaturediscontinuity means becomes sufficiently small in effect when saidpressure has dropped to a predetermined level to permit said secondarycryopanel to attain cryopumping temperature.

12. A cryopump in accordance with claim 11 wherein said refrigeratingmeans is a mechanical cryogenic refrigerator.

13. A cryopump in accordance with claim 11 wherein 9 10 said cryopanelsare copper and said temperature discon- 3,130,562 4/1964 Wood et a1.6255.5 finvlty means higlrpurity pp 3,137,551 6/1964 Mark 62-555References Cited 7 3,220,167 11/1965 Ster et a1. 6255.5

UNITED STATES PATENTS 5 LLOYD L. KING, Primary Examiner.

3,122,896 3/1964 Hickey 62-555

1. A CRYOPANEL ARRAY SUITABLE FOR INCORPORATION INTO A CRYOPUMP,COMPRISING IN COMBINATION (A) A PRIMARY CRYOPANEL IN THERMAL CONTACTWITH REFRIGERANT MEANS; (B) AT LEAST ONE SECONDARY CRYOPANEL HAVING ACONDENSATION SURFACE AREA AT LEAST AS GREAT AS THAT OF SAID PRIMARYCRYOPANEL, AND SEPARATED FROM SAID PRIMARY CRYOPANEL BY (C) TEMPERATUREDISCONTINUITY MEANS RESPONSIVE TO A DECREASE IN PRESSURE WITHIN SAIDCYROPUMP WHEREBY SAID TEMPERATURE DISCONTINUITY MEANS BECOMESSUFFICIENTLY SMALL IN EFFECT WHEN SAID PRESSURE HAS DROPPED TO APREDETERMINED LEVEL TO PERMIT SAID SECONDARY CRYOPANEL TO ATTAINCRYOPUMPING TEMPERATURE.